In manufacturing various components such as component for automobiles and component for construction machines, a spheroidizing annealing treatment is usually applied with the purpose of imparting cold workability to a hot-rolled material such as carbon steel and alloy steel. The rolled material after spheroidizing annealing is subjected to cold working, then to machining such as cutting work for forming the material into a predetermined shape, and further to a quenching-tempering treatment for final strength adjustment.
In recent years, from the viewpoint of energy saving, the conditions of spheroidizing annealing are reexamined, and among others, it is demanded to shorten the spheroidizing annealing time. If the time for a soaking treatment in spheroidizing annealing can be reduced to half or less, this can be expected to provide sufficient energy saving.
Shortening of the spheroidizing annealing treatment indicates, for example, to reduce the soaking treatment time from 6 hours to 3 hours or less. In the case of using a conventional steel for a mechanical structure for cold rolling, it is known that when the spheroidizing annealing time is shortened, spheroidization of a carbide cannot be achieved sufficiently.
Several proposals have been made so far with respect to the method for manufacturing a steel wire rod capable of achieving rapid spheroidizing annealing. For example, Patent Document 1 discloses a method for manufacturing a steel wire rod capable of achieving rapid spheroidization, the method including hot finish rolling and then cooling to a range of 600 to 650° C. at a cooling rate of 5° C./sec or more. However, in this technique, the cooling rate in the temperature region of approximately from 720 to 650° C. allowing production/growth of pro-eutectoid ferrite is high (paragraph 0043, etc. of Patent Document 1), and it is considered that refinement of the pro-eutectoid ferrite or increase in the aspect ratio is caused to incur refinement of the microstructure after spheroidizing annealing and in turn, occurrence of hardening due to grain refinement and softening becomes insufficient.
Patent Document 2 discloses a manufacturing method of a steel for a mechanical structure for cold working, the method including finish rolling, cooling to a temperature range of 640 to 680° C. at an average cooling rate of 5° C./sec or more, and cooling for 20 seconds or more at an average cooling rate of 1° C./sec or less. However, since the subsequent cooling condition is standing to cool to room temperature (paragraph 0040 of Patent Document 2), it is considered that refinement of pearlite is insufficient and when the spheroidizing annealing time is shortened, insufficient spheroidization is caused.
Patent Document 3 discloses a manufacturing method of a steel for cold heading, the method including hot rolling and after the termination of rolling, cooling at a cooling rate of 1° C./sec or less. However, since the cooling is very slow cooling even in the temperature region where pearlite precipitation occurs (paragraph 0022 of Patent Document 3), it is considered that the pearlite lamellar interval becomes coarse and when the spheroidizing annealing time is shortened, a sufficiently spheroidized microstructure is not obtained.